1. Technical Field of the Invention
The present invention relates to a steering apparatus for a vehicle that directs front and rear steerable wheels in response to operator input in which the steerable wheels are not mechanically coupled to a manually steerable member.
2. Description of the Prior Art
A typical automotive vehicle is steered by transmitting operations of a manually steerable member, such as a steering wheel, to a steering mechanism for directing steerable wheels for steering. Generally, the manually steerable member is located inside the vehicle passenger compartment, and the steerable wheels are located at the front and rear of the vehicle. Thus, a suitable steering mechanism is necessary to couple the manually steerable member and the front and rear steerable wheels.
A representative steering mechanism is a rack-and-pinion type steering mechanism. In a rack-and-pinion steering mechanism, the rotational motion of the steering wheel is communicated through a steering column to a pinion gear at its distal end. The pinion gear is engaged with a rack gear disposed laterally between the steerable wheels, which in turn are coupled to the rack gear by knuckle arms and tie rods. In this manner, rotation of the steering wheel is translated into the lateral movement of the rack gear, which causes the steerable wheels to pivot in the desired direction. In general, mechanical steering mechanisms are power-assisted by hydraulic or electrical assist units.
In order to overcome limitations presented by mechanical steering systems, it has been proposed to utilize a steering system in which the manually steerable member is not mechanically coupled to the steerable wheels and steering movement is achieved by an electrically controlled motor, a steer-by-wire system. In a steer-by-wire system, a road wheel actuator operates in response to detected values of various steering parameters, such as for example steering wheel angle, vehicle speed, vehicle lateral acceleration, and road wheel angle. The detected values are communicated electronically to the road wheel actuator from sensors, or alternatively, from a centralized controller. Upon receipt and processing of the steering command, the road wheel actuator orients the steerable wheels in the desired direction in accordance with the vehicle steering parameters.
In order to provide a steering “feel” to the vehicle operator, a typical steer-by-wire vehicle will also utilize a reaction torque generator that synthesizes and generates a reaction torque in the manually steerable member. For example, if the manually steerable member is a steering wheel, then the reaction torque generator will generally rotate a shaft coupled to the steering wheel in order to give the vehicle operator a resistive or assisting torque. In general, the magnitude and direction of the reaction torque will be determined by a control system cooperating between the reaction torque generator, the road wheel actuator, and the various vehicle sensing systems.
The adaptability of steer-by-wire systems to myriad situations provides a great number of advantages not apparent in a mechanically steered vehicle. It is believed that a hybrid-type steering system is needed in order to ease the transition in the automotive market from mechanically coupled steering systems to steer-by-wire steering systems.
Moreover, rear wheel steering is becoming more and more common on vehicles. Front and Rear wheel steering provides increased vehicle maneuverability relative to front wheel steering only vehicles. Vehicle front and rear wheel steering offers advantages over vehicles having only front wheel steering. However, manufacturers have been challenged in implementing rear wheel steering with other vehicle features.